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The Science of Breeding

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Breeder's Handbook: Lactation

By Dr. Bretaigne Jones
Senior Scientific Communications Manager

The most nutritionally stressful period in the life of a dog is while nursing a litter of puppies. Depending on the size of the litter, the bitch’s nutritional needs can be 300 percent of her normal non-pregnant maintenance need at the peak of lactation.

The nursing puppies are totally dependent upon the mother’s milk for the first four weeks of life. It is vitally important that the quality and quantity of the bitch’s milk be optimal throughout lactation. There are many factors that can directly affect lactation, both positively and negatively.

The dog’s mammary glands run in series on each side of the abdominal midline, usually paired from rib cage to inguinal area. The number can vary from 8 to 12 regardless of breed, with four to six mammary glands on either side of the midline. Smaller breeds tend to have four pairs, while larger breeds average five.

Each teat will have 8 to 20 openings on the blunt end. Each opening presents an opportunity for bacterial contamination. Due to the nature of milk, it makes an excellent growth media for bacteria, and infection spreads quickly into the glandular tissue. Inflammation of the mammary tissue is called mastitis. A clean environment is important to help prevent mastitis, and to ensure the health of the puppies.

Interestingly, the tissue within the mammary gland that secretes the milk is present only during times of need, such as pregnancy, during lactation, and continuing for about 45 days after the pups are weaned. It is also present during pseudopregnancy, which accounts for the presence of some milk in the glands in this circumstance. At other times, the tissue atrophies and the cells phase out.

Successful lactation is the combined effect of two actions, the secretion of milk by the mammary tissue and the removal of accumulated milk from the glands and teats by suckling. Both are necessary components to maintain lactation until the puppies can start the weaning process and gradually eat other food.

Hormones play a key role in the preparation of the mammary glands for milk production before whelping, and for production throughout lactation. Likewise, the suckling of pups empties the reservoirs of milk, and signals the brain that more is needed. If suckling stops, or is greatly reduced, the engorgement of the teats and glands signals the brain to decrease or stop the production of milk. The interaction between these two components are vital, as neither one can maintain lactation by itself.

There are four hormones that work to stimulate milk production and two that influence milk let-down responding to suckling behavior. The production stimulant hormones are estrogen, progesterone, prolactin, and relaxin; while the let-down hormones are prolactin and oxytocin.

Milk Production
Estrogen is produced primarily by the ovaries. It signals the mammary tissue of anticipated pregnancy. This initiates the production of glandular cells that will then secrete milk when lactation begins. If pregnancy occurs, estrogen is also secreted by the placentas. Estrogen also affects lactation by its interactions with prolactin, another hormone.

Progesterone, another ovarian hormone, also impacts mammary development during pregnancy and helps maintain active secretion of milk during lactation. Progesterone does depend on estrogenic activity to accomplish its affect in the milk glands.

Prolactin is a hormone primarily secreted by the pituitary gland, but it is believed that secondary sources are the placenta and uterine tissue. During the last half of pregnancy, prolactin supports the luteal cells in the ovary, so that the level of progesterone secreted by those cells is maintained in order to continue the pregnancy. As whelping nears, prolactin begins to prepare the mammary gland for production, and continues production activity throughout lactation. An important role of prolactin aside from milk production is the stimulation of normal maternal behavior.

Relaxin is the fourth hormone involved in lactation. It is only present during pregnancy and lactation, and thus can be used for pregnancy diagnosis. Relaxin is produced by the placenta, beginning about mid-gestation. Its role is to prepare the pregnant uterus, cervix and vagina as whelping nears. In conjunction with estrogen, relaxin stimulates mammary development.
The two hormones that influence milk-letdown are oxytocin and prolactin. Stimulation of the teat by rooting or suckling behavior releases both hormones. Oxytocin is the primary agent and reacts at the cellular level in the mammary gland to contract and squeeze milk down into the teats. Prolactin acts in a much more muted fashion by helping the secretory cells to release the milk into the reservoir areas.

When whelping labor begins, the puppy closest to the cervix is pressed against it. Ideally the pup is presenting nose first. The pressure of the pup’s nose into the cervix stimulates the release of prolactin, initiating milk production. When a bitch is delivered by cesarean surgery, especially if the surgery is prior to the onset of labor, the cervix is not pressured by contractions moving the puppies forward. This lack of cervical stimulation may account for delayed milk, or lack of milk, following surgical delivery.

Prolactin release resulting from cervical stimulation also initiates maternal behavior. In the absence of this, bitches delivered by C-section may be slower to take care of the pups upon waking, occasionally even showing aggressive behavior towards her pups.

Milk Nutrients
The first secretions from the mammary tissue are rich in immunoglobulins, transferring passive immunity to the pups for a period of time. Colostrum, present only the first few days of lactation, is slightly more viscous than regular milk, and may have a yellowish tint. Ordinarily, the cells lining the intestinal tract are too impermeable to allow absorption of intact immunoglobulins, and they would be broken down to component amino acids by enzymes. However, in newborns, the intestines are highly permeable for up to 24 hours, allowing these intact immunoglobulins into circulation immediately. This provides the first immunological defenses to be active immediately.

The bitch’s milk has all the nutrients needed to support life and growth in her pups for the first four weeks. Just as the puppies’ rate of growth and level of activity change during this time, so do the components of the milk. The protein portion will gradually increase from approximately 4.5 to 6.5 percent over six weeks. The fat content will also increase initially, from approximately 2.5 to 4.5 percent during the first three weeks, then returning to the original level. Calcium is another nutrient that progressively increases, matching the needs of the rapidly growing puppies.

While nutrient composition is important, the water content is equally so. Newborn puppies experience a high rate of water turnover. This constant intake of fluid maintains the puppy’s blood volume and hydration status. The nursing bitch needs considerably more water than normal for the entire time that pups are suckling. Lack of water will decrease her milk production and may cause her to dehydrate, as well as the puppies.
Mastitis is a constant concern for lactating dogs. There is significant opportunity for bacterial contamination of the teats from the environment as well as from the nails of the nursing puppies as they knead the mammary tissue while nursing. Depending on the type of bacteria initiating mastitis, the milk may or may not be suitable for the pups to ingest. Not all cases of mastitis have a bacterial infection associated. Additionally, mastitis may be localized to one area, or may spread to adjacent glands. Sometimes the bitch will show symptoms of mastitis such as reddened, inflamed mammary glands, and discomfort or pain when the pups try to nurse. Milk from mastitic glands may have a different color than normal, such as greenish tint, or blood tinged. Mastitis can develop rapidly, without symptoms. The first the breeder knows of a problem is puppy deaths, usually due to lack of milk intake resulting in dehydration and hypoglycemia. This type of development can be life-threatening for the bitch as well.

The decision on whether or not to take the pups and hand-feed varies with each individual situation. Variables such as the extent of the mastitis, the health of the dam, and the cause of the mastitis will all influence the decision.

When the mammary glands don’t produce milk at all or at an inadequate level, it is usually secondary to some other problem. The total lack of milk is called agalactia. More common is decreased milk production. One potential cause can be that the nesting area is too warm and the bitch cannot sufficiently get away from the heat to cool down periodically. The bitch may not be getting enough food or water, or the food is not the right type (such as feeding a maintenance diet instead of an appropriate puppy diet). Mastitis can decrease milk production, as can infection within the uterus (metritis). Stress from overcrowding, too much noise, or too much traffic through the maternity area can effectively shut down milk production. The first-time or nervous mother can be stressed to the point of shutting down milk production as well. Some breeds, or familial lines within breeds, are just not good milk producers. If the dam has poor milk production with multiple litters, the female pups may likewise be poor producers in the future. Milk production is a genetic trait.

There are some treatments that may help, depending on the initial cause. While oxytocin has been recognized for stimulating milk letdown, it is not always the answer because it does not influence production at all. If the glandular cells are not producing milk, there will be no milk to let down. Drugs such as metoclopramide, domperidone, or low-dose acepromazine (in the case of nervous bitch, however will also cause sedation) may help. Many veterinarians report success with acupuncture therapy.

In any event, the cause of the agalactia or decreased production needs to be determined to facilitate effective treatment. It is important to seek guidance from a veterinarian for appropriate options. If infection is present, it must be addressed as soon as possible.
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